Double tap the home button. ;)

Unfortunately Sony disabled it. When I asked about it they said oreo should handle multiple apps better than nougat so it's not needed... But i used it for task switching and haven't found an alternative.

It’s convoluted but you can do it. This is from memory but it’s something like this:

• Hard press the home button to bring up the app menu.
• Highlight the app you want to kill and hard press the select button.
• Then you’ll get a pop-up menu about that app. Choose “info”.
• Then choose “force kill”.